Photodissociation of S2 (X3Σg-, a1Δg, and b1Σg+) in the 320-205 nm Region.

Photodissociation of vibrationally and electronically excited sulfur dimer molecules (S2) has been studied in a combined experimental and computational quantum chemistry study in order to characterize bound-continuum transitions. Ab initio quantum chemistry calculations are carried out to predict the potential energy curves, spin-orbit coupling, transition moments, and bound-continuum spectra of S2 for comparison with the experimental data. The experiment uses velocity map imaging to measure S-atom production following S2 photoexcitation in the ultraviolet region (320-205 nm). A pulsed electric discharge in H2S produces ground-state S2 X3Σg-(v = 0-15) as well as electronically excited singlet sulfur and b1Σg+(v = 0, 1), and evidence is presented for the production and photodissociation of S2 a1Δg. In a previous paper, we reported threshold photodissociation of S2X3Σg-(v = 0) in the 282-266 nm region. In the present study, S(3PJ) fine structure branching and angular distributions for photodissociation of S2 (X3Σg-(v = 0), a1Δg and b1Σg+) via the B″3Πu, B3Σu- and 11Πu excited states are reported. In addition, photodissociation of the X3Σg-(v = 0) state of S2 to the second dissociation limit producing S(3P2) + S(1D) is characterized. The present results on S2 photodynamics are compared to those of the well-studied electronically isovalent O2 molecule.

Imaging multiphoton ionization and dissociation of rotationally warm CO via the B+Σ1 and EΠ1 electronic states.

Pathways for formation of C+ and O+ ions when applying (2 + 1) resonance enhanced multiphoton ionization (REMPI) of CO via the B1Σ+ and E1Π electronic states are characterized with the velocity map imaging technique. By employing an unskimmed pulsed valve, it was possible to obtain sharp images for a wide range of initial CO J-states. Most of the atomic ion production pathways could be assigned as one- or two-photon dissociation of a series of vibrational levels of the CO+ X2Σ+ and A2Π states. Large enhancements in dissociation of particular CO+ vibrational states in these progressions could be accurately assigned to accidental resonances of the REMPI laser with CO+ X2Σ+-B2Σ+ transitions.

A simple resonance enhanced laser ionization scheme for CO via the A1Π state.

We investigate the laser ionization process taking place when the CO molecule is exposed to vacuum ultraviolet (VUV) radiation resonant with the CO A1Π(v = 0) ← X1Σ+(v = 0) transition around 154 nm, along with the ultraviolet (UV) and visible (Red) radiation used to generate VUV by four-wave difference-frequency mixing. By measuring the CO+ ion recoil and a room temperature gas spectrum, it is possible to assign the ionization process as 1 + 1' + 1'' REMPI where the one-photon steps refer to the VUV, UV, and Red radiation, respectively. Resonance enhanced ionization of rotational states around J = 12 arise due to the overlap of the fixed wavelength UV (∼250 nm) with the R band-head of a transition assigned to CO E1Π(v = 6) ← A1Π(v = 0) with a term value of 104 787.5 cm-1. The REMPI process is efficient and polarization sensitive and should be useful in a wide range of studies involving nascent CO.

A new high intensity and short-pulse molecular beam valve.

In this paper, we report on the design and performance of a new home-built pulsed gas valve, which we refer to as the Nijmegen Pulsed Valve (NPV). The main output characteristics include a short pulse width (as short as 20 µs) combined with operating rates up to 30 Hz. The operation principle of the NPV is based on the Lorentz force created by a pulsed current passing through an aluminum strip located within a magnetic field, which opens the nozzle periodically. The amplitude of displacement of the opening mechanism is sufficient to allow the use of nozzles with up to 1.0 mm diameter. To investigate the performance of the valve, several characterizations were performed with different experimental methods. First, a fast ionization gauge was used to measure the beam intensity of the free jet emanating from the NPV. We compare free jets from the NPV with those from several other pulsed valves in current use in our laboratory. Results showed that a high intensity and short pulse-length beam could be generated by the new valve. Second, the NPV was tested in combination with a skimmer, where resonance enhanced multiphoton ionization combined with velocity map imaging was used to show that the NPV was able to produce a pulsed molecular beam with short pulse duration (~20 µs using 0.1% NO/He at 6 bars) and low rotational temperature (~1 K using 0.5% NO/Ar at 6 bars). Third, a novel two-point pump-probe method was employed which we label double delay scan. This method allows a full kinematic characterization of the molecular beam, including accurate speed ratios at different temporal positions. It was found that the speed ratio was maximum (S = 50 using 0.1% NO/He at 3 bars) at the peak position of the molecular beam and decreased when it was on the leading or falling edge.

State-to-state differential and relative integral cross sections for rotationally inelastic scattering of H2O by hydrogen.

State-to-state differential cross sections (DCSs) for rotationally inelastic scattering of H(2)O by H(2) have been measured at 71.2 meV (574 cm(-1)) and 44.8 meV (361 cm(-1)) collision energy using crossed molecular beams combined with velocity map imaging. A molecular beam containing variable compositions of the (J = 0, 1, 2) rotational states of hydrogen collides with a molecular beam of argon seeded with water vapor that is cooled by supersonic expansion to its lowest para or ortho rotational levels (J(KaKc) = 0(00) and 1(01), respectively). Angular speed distributions of fully specified rotationally excited final states are obtained using velocity map imaging. Relative integral cross sections are obtained by integrating the DCSs taken with the same experimental conditions. Experimental state-specific DCSs are compared with predictions from fully quantum scattering calculations on the most complete H(2)O-H(2) potential energy surface. Comparison of relative total cross sections and state-specific DCSs show excellent agreement with theory in almost all details.

Communication: Mapping water collisions for interstellar space conditions.

We report a joint experimental and theoretical study that directly tests the quality of the potential energy surfaces used to calculate energy changing cross sections of water in collision with helium and molecular hydrogen, at conditions relevant for astrophysics. Fully state-to-state differential cross sections are measured for H(2)O-He and H(2)O-H(2) collisions at 429 and 575 cm(-1) collision energy, respectively. We compare these differential cross sections with theoretical ones for H(2)O+H(2) derived from state-of-the-art potential energy surfaces [P. Valiron et al., J. Chem. Phys. 129, 134306 (2008)] and quantum scattering calculations. This detailed comparison forms a stringent test of the validity of astrophysics calculations for energy changing rates in water. The agreement between theory and experiment is striking for most of the state-to-state differential cross sections measured.

REMPI spectroscopy and predissociation of the C(1)B(1)(v = 0) rotational levels of H(2)O, HOD and D(2)O.

Rotational analysis of the (2 + 1) resonance enhanced multiphoton ionization (REMPI) spectrum of the C(1)B(1) Rydberg state of the water isotopomers H(2)O, HOD and D(2)O is reported. Spectroscopic parameters for the v = 0 vibrational level of the C(1)B(1) state of the mixed isotopomer HOD are derived and its spectra are accurately simulated for the first time using the PGOPHER program. Simulation of two photon spectra of the C(1)B(1)-X(1)A(1) transition of HOD requires two transition moments, and the ratio of these is determined and explained by a simple geometrical model. Optimal transitions for state-selective detection of low energy rotational states are identified for all three molecules. Analysis of the linewidths in the present work, combined with previous work [H. H. Kuge and K. Kleinermanns, J. Chem. Phys., 1989, 90, 46-52; K. J. Yuan et al., Proc. Natl. Acad. Sci. U. S. A., 2008, 105, 19148-19153; M. N. R. Ashfold et al., Chem. Phys., 1984, 84, 35-50; G. Meijer et al., J. Chem. Phys., 1986, 85, 6914-6922.], suggests that while a simple ⟨J(a)'(2)〉-dependent model for heterogeneous predissociation of the C(1)B(1) Rydberg state accounts for much of the quantum number dependence, it is not sufficient for describing the predissociation in any of the three isotopomers. The component of the linewidth due to the homogeneous predissociation attributed to predissociation of the C(1)B(1) by the Ã(1)B(1) state was found to be significantly narrower than in previous work, indicating a longer lifetime of the C(1)B(1) Rydberg state. The current work provides the basis for on-going studies of rotational energy transfer in the mixed isotopomers of water using the velocity map imaging technique.

Photodissociation dynamics of the A 2Sigma+ state of SH and SD radicals.

Atomic sulfur products from predissociation of the lowest rotational states of SH/SD A (2)Sigma(+) (v(')=0,1,2) are studied using velocity map imaging. The dissociation process, which is slow compared to rotation, is dominated by interference effects due to predissociation of states with low rotation quantum numbers prepared by photoexcitation using overlapping transitions of different parities. The measured product angular distributions can be modeled using the methods presented recently by Kim et al. [J. Chem. Phys. 125, 133316 (2006)]. The S((3)P(J)) (2+1) resonance enhanced multiphoton ionization scheme used in the detection step of the experiment is sensitive to the angular momentum polarization of the atomic fragments. S((3)P(J)), J=2,1,0, fine-structure yields, angular distributions, and atom polarization parameters are reported. Strong polarization of the S((3)P(2,1)) products was observed along with a weak sensitivity of the branching ratio to excess energy and a full insensitivity of the atomic product polarization to excess energy. None of the data fit the predictions of either adiabatic or diabatic photodissociation, emphasizing the need for a fully quantum treatment.

State-to-state inelastic scattering of OH by HI: a comparison with OH-HCl and OH-HBr.

Relative state-to-state cross sections and steric asymmetries have been measured for the scattering process: OH (X (2)Pi(32),v=0,J=32,M(J)=32,f)+HI ((1)Sigma,v=0,J<4)-->OH (X (2)Pi,v=0,Omega=12,J=12-52 and Omega=32,J=32-92,ef)+HI, at 690 cm(-1) collision energy. Comparison with the previously studied systems OH-HCl and OH-HBr reveals relevant features of the potential energy surfaces of these molecular systems. Some measured differences concerning the internal energy distribution after collision and the propensities for the impact with one or the other side of the OH molecule in scattering by HCl, HBr, and HI molecules are discussed.

Rotationally inelastic scattering of OH (2Pi 3/2, v=0, J=3/2, f) by HBr (1Sigma, v=0, J<4).

Relative state-to-state cross sections of OH molecules in the (2)Pi(32), v=0, J=32, M(J)=32, f state have been determined for transitions up to (2)Pi(32), v=0, J=112, f and (2)Pi(12), v=0, J=72, e states by collisions with HBr molecules ((1)Sigma, v=0, J<4) at 750 cm(-1) collision energy. In order to investigate features of the anisotropy of the OH-HBr potential energy surface, the steric asymmetries, which account for the effect of the OH orientation with respect to the collision partner, have been measured. A comparison with other systems previously studied shows strong similarities with the OH-HCl system.

Photofragment alignment in the photodissociation of I2 from 450 to 510 nm.

A combination of velocity map imaging and slicing techniques have been used to measure the product recoil anisotropy and angular momentum polarization for the photodissociation process I2-->I(2P(3/2))+I(2P(3/2)) and I2-->I(2P(3/2)))+I(2P(1/2)) in the 450-510 nm laser wavelength region using linearly polarized photolysis and probe laser light. The former channel is produced predominantly via perpendicular excitation to the 1Piu state, and the latter is predominantly parallel, via the B 3Pi(0u)+ state. In both cases we observe mostly adiabatic dissociation, which produces electronically aligned iodine atoms in the mid /m/=1/2 states with respect to the recoil direction.

Iron monoxide photodissociation.

The photodissociation of (56)FeO was studied by means of the velocity map imaging technique. A molecular beam of iron atoms and iron monoxide molecules was created using an electrical discharge with an iron electrode in a supersonic expansion of molecular oxygen. The ground state iron atom Fe((5)D(4)) and FeO concentrations in the molecular beam have been estimated. The dissociation energy of the FeO X (5)Delta ground electronic state was found to be D(0) (0)(FeO)=4.18+/-0.01 eV. The effective absorption cross section of FeO at 252.39 nm (vac), leading to the Fe((5)D(4))+O((3)P) dissociation channel, is approximately 1.2 x 10(-18) cm(2). A (1+1) resonantly enhanced multiphoton ionization spectrum of (56)FeO in the region 39 550-39 580 cm(-1) with rotational structure has been observed, but not assigned. Angular distributions of Fe((5)D(4)) and Fe((5)D(3)) products for the channel FeO-->Fe((5)D(4,3))+O((3)P) have been measured at several points in the 210-260 nm laser light wavelength region. The anisotropy parameter varies strongly with wavelength for both channels.

Observation of direct dissociative ionization in molecular hydrogen.

Direct dissociative ionization is the simplest three-body breakup process in H2. We describe the experimental verification of direct dissociative ionization to the repulsive A2Sigma(+)(u) state by resolving the kinetic energy and angular distributions of the formed protons. A (2+1) resonant enhanced multiphoton ionization process via the isotropic E,F (1)Sigma(+)(g)(v = 6,J = 0) level is employed. The structure in the kinetic energy spectrum is well described by a projection of the vibrational wave function of the E,F (1)Sigma(+)(g)(v = 6,J = 0) state onto the repulsive ionic state. The electronic character of the ionization continuum is revealed by the proton angular distribution.

Laser photochemistry of molecular oxygen.

Molecular oxygen poses very difficult challenges in molecular photochemistry due to the extreme weakness of its ultraviolet absorption spectrum. In the past few years a new technique called velocity map imaging has been able to overcome many of the experimental obstacles to the study of this all-important atmospheric species. In this Account several aspects of laser photochemistry of O(2) will be described, including the unraveling of the Herzberg and Schumann-Runge continua using velocity map imaging.

Photoacoustic spectroscopy using quantum-cascade lasers.

Photoacoustic spectra of ammonia and water vapor were recorded by use of a continuous-wave quantum-cascade distributed-feedback (QC-DFB) laser at 8.5 mum with a 16-mW power output. The gases were flowed through a cell that was resonant at 1.6 kHz, and the QC-DFB source was temperature tuned over 35 nm for generation of spectra or was temperature stabilized on an absorption feature peak to permit real-time concentration measurements. A detection limit of 100 parts in 10(9) by volume ammonia at standard temperature and pressure was obtained for a 1-Hz bandwidth in a measurement time of 10 min.

Dynamics of Acetaldehyde Production during Anoxia and Post-Anoxia in Red Bell Pepper Studied by Photoacoustic Techniques.

Acetaldehyde (AA), ethanol, and CO2 production in red bell pepper (Capsicum annum L.) fruit has been measured in a continuous flow system as the fruit was switched between 20% O2 and anaerobic conditions. Minimum gas phase concentrations of 0.5 nL L-1, 10 nL L-1, and 1 mL L-1, respectively, can be detected employing a laser-based photoacoustic technique. This technique allows monitoring of low production rates and transient features in real time. At the start of anaerobic treatment respiration decreases by 60% within 0.5 h, whereas AA and ethanol production is delayed by 1 to 3 h. This suggests a direct slow-down of the tricarboxylic acid cycle and a delayed onset of alcoholic fermentation. Reexposure of the fruit to oxygen results in a 2- to 10-fold upsurge in AA production. A short anoxic period leads to a sharp transient peak lasting about 40 min, whereas after numerous and longer anoxic periods, post-anoxic AA production stays high for several hours. High sensitivity of the fruit tissue to oxygen is further evidenced by a sharp decrease in post-anoxic AA production upon an early return to anaerobic conditions. Ethanol oxidation by the "peroxidatic" action of catalase is proposed to account for the immediate post-anoxic AA upsurge.

Molecular analysis by ionization of laser-desorbed neutral species.

A powerful molecular surface analysis technique for the analysis of complex materials, such as polymer/additive systems, consists of laser desorption of surface molecules and subsequent ionization of these gas-phase molecules with resonant or nonresonant laser ionization. These molecular ions are subsequently detected by Fourier-transform mass spectrometry or time-of-flight mass spectrometry. We show that different wavelengths for the postionization step permit selectivity that provides important additional information on the chemical makeup of these complex materials. Near-UV wavelengthsselectively ionize aromatic polymer additives, far-UV wavelengths photoionize other nonaromatic species; and vacuum-UV wavelengths provide access to all the desorbed species. In addition to these applied results, we study many fundamental issues of laser desorption, such as desorption thresholds, velocity distributions, postionization wavelength selectivity, etc. The Fourier-transform mass spectrometer and time-of-flight mass spectrometer are discussed in detail.

Noninvasive determination of effective (nonshunted) pulmonary blood flow in normal and injured lungs.

PEEP is utilized in acute respiratory failure to decrease intrapulmonary shunting and improve oxygenation. Despite these beneficial effects, PEEP may adversely affect cardiac output, thus reducing oxygen delivery. To monitor some of the cardiopulmonary effects of PEEP, we utilized a noninvasive rebreathing technique to measure effective (nonshunted) pulmonary blood flow (Qepr) and compared the results to those measured by thermodilution (Qepi) in normal and oleic acid-injured canine lungs. Qepr was highly correlated with Qepi (r = .92, r2 = .85, p less than .001) despite large variations in PEEP before lung injury (0 to 15 cm H2O) and after lung injury (0 to 20 cm H2O). This close correlation was found even with wide ranges in cardiac output (1.01 to 6.45 L/min) and intrapulmonary shunt fractions (0.03 to 0.67). This technique may prove valuable as a noninvasive method by which to monitor and adjust PEEP therapy in patients with acute lung injury.

Response of pulmonary mechanics to terbutaline in patients with bronchopulmonary dysplasia.

Terbutaline was administered to eight ventilator-dependent infants with evolving bronchopulmonary dysplasia to determine the potential for reductions in airway resistance. The drug was administered by nebulization through the endotracheal tube. Pre-treatment and post-treatment pulmonary mechanics measurements were performed. Patients had reductions in inspiratory (-14%, p = NS), expiratory (-47%, p less than .05) and total (-24%, p = NS) resistances. Tidal volume increased 36% (p less than .01). This study demonstrates that nebulized terbutaline can improve pulmonary mechanics in ventilator-dependent infants with evolving bronchopulmonary dysplasia.